The present invention relates to a liquid jetting apparatus such as an ink jet printer and a method of driving the same. Particularly, the present invention relates to an apparatus and a method for driving piezoelectric elements provided with a print head in an ink jet printer, so that ink droplets are ejected from nozzle orifices formed with the print head.
An ink jet color printer of a type in which ink of several colors is ejected from a print head has spread up to now, and it has been widely used in order to print images processed by a computer with multi-colors and multi-tones.
For example, in an ink jet printer using a piezoelectric element as a drive element for ink ejection, plural piezoelectric elements associated with nozzles are selectively driven thereby to generate dynamic pressure to eject ink droplets from the nozzles. Printing is performed such that the ink droplets are landed on a print sheet to form ink dots thereon.
Each piezoelectric element is driven by a drive signal supplied from a driver circuit (driver IC) mounted in a printer body or a print head thereby to eject the ink droplets from the nozzles.
When the piezoelectric element is not driven (that is, when the printing is not performed), electric charges accumulated therein are discharged by inherent insulation resistance, so that a thus lowered potential of the piezoelectric element happens to affect the ink ejection.
In view of the above, Japanese Patent No. 3097155 discloses a head driving apparatus and a head driving method, in which charging voltage is applied to piezoelectric elements in accordance with charge signals when the piezoelectric elements are not driven, in order to keep a charged potential.
To drive the print head in such a way, a drive signal applied to each piezoelectric element is so configured as to have a high potential for deactivating the piezoelectric element and to have a lower potential for activating the same. Therefore, consumed power becomes large and the voltage applied to the piezoelectric element becomes relatively high, so that voltage drop due to the discharge (i.e., power loss) is also becomes large.
Increasing the number of piezoelectric elements arranged in a unit area is increased to improve the print quality, the distance between adjacent piezoelectric elements is accordingly reduced. In a case where an activated element and a deactivated element are juxtaposed, discharging between the adjacent elements would occur because of a potential difference caused by the voltage drop.
In the above case, the breakdown voltage of each element becomes low. Therefore, in a case where the drive signal having the maximum voltage higher than the breakdown voltage is applied to such an element, desired operation would not be attained. To avoid such a situation, it is necessary to apply insulation processing between the adjacent elements (e.g., filling an insulating material).
In a case where a charging voltage is suddenly applied to the piezoelectric element in which such voltage drop is occurred, there is a probability that the element happens to be driven so that ink drops are ejected unintentionally. To avoid such a situation, it is necessary to consider the timing of applying the charge signal when designing the drive signal.
It is therefore an object of the invention to provide, with simple configuration, an apparatus and a method for driving a print head in an ink jet printer, which lowers a potential difference between electrodes of each piezoelectric element, and reduces a voltage drop occurring therein due to discharging, while eliminating erroneous operations thereof.
In order to achieve the above object, according to the present invention, there is provided a head driving apparatus, incorporated in an ink jet printer which comprises:
a print head, provided with a plurality of nozzles;
piezoelectric elements, each associated with one of the nozzles and provided with a drive electrode and a common electrode; and
a head driver, which generates a drive signal for driving the piezoelectric elements, and selectively supplies the drive signal to at least one of the piezoelectric elements to eject an ink droplet from at least one associated nozzle, the head driving apparatus comprising:
a bias power source, which applies a bias voltage having a predetermined potential to the common electrode of each piezoelectric element.
In this apparatus, by directly applying the bias voltage to the common electrode of the piezoelectric element from the bias power source, the potential of the piezoelectric element is held at the bias voltage. Consequently, since the voltage applied between both electrodes of the piezoelectric element becomes relatively low, consumed power is reduced.
Further, since the predetermined bias voltage is always applied to the common electrode of the piezoelectric element, leak current is reduced even if natural discharge of the piezoelectric element occurs, so that the voltage drop is reduced. Therefore, not only power loss is reduced, but also the steep voltage variation can be avoided when the piezoelectric element is charged so that the occurrence of the erroneous operation of the piezoelectric element can be eliminated. In addition, the restriction on the waveform design for placing the charge signal in the drive signal can be relaxed.
Further, since the voltage applied to the piezoelectric element becomes relatively low, occurrence of the discharge due to the voltage difference between the driven piezoelectric element and the non-driven piezoelectric element is also reduced. Even if the number of the piezoelectric elements per a unit area is increased while each size of the piezoelectric element is made small (the breakdown voltage becomes low), the piezoelectric element can normally operate without performing the insulation processing between the electrodes of the piezoelectric elements.
Preferably, the potential of the bias voltage is variable.
In this apparatus, the bias voltage can be controlled in accordance with the reference potential of the drive signal applied to the piezoelectric element which is inherent of each ink jet printer. Therefore, the voltage applied between both electrodes of each piezoelectric element can be set lower.
Preferably, the bias power source is provided as a logic power source.
In this apparatus, the bias power source can be constituted simply, readily and at a low cost.
Preferably, the bias power source generates the bias voltage based on a power supplied from a power source for driving the print head.
In this apparatus, since the bias voltage is generated using the existing head driving power source, it is not necessary to provide, for example, a logic power source, and the bias voltage can be obtained by the simple construction and at a low cost.
Here, it is preferable that the bias power source includes: a condenser, electrically connected to the common electrode; and a constant-voltage circuit, which applies the bias voltage to the condenser.
In this apparatus, the potential of the common electrode of the piezoelectric element is held at the bias voltage applied from the condenser.
Further, it is preferable that the constant-voltage circuit includes a Zener diode, a current limiting resistance and a coupling element. The Zener diode is electrically connected to the head driving power source through the current limiting resistance. The Zener diode is electrically connected to the common electrode through the coupling element.
In this apparatus, the condenser is charged by the stable bias voltage, and it is prevented by the coupling element that the electric charges discharged from the common electrode from flowing to the Zener diode.
Still further, it is preferable that the constant-voltage circuit includes a discharging diode electrically connected to the head driving power source in parallel with the current limiting resistance, such that a current is flowed to the head driving power source through the discharging diode.
In this apparatus, in a case that the potential of the head driving power source becomes to zero due to deactivation or the like, the electric charge charged in the condenser bypasses the current limiting resistance and is discharged through the discharging diode, whereby the condenser can be discharged quickly.
Preferably, the bias power source includes: a first condenser, electrically connected to the common electrode; and a charger, which charges the first condenser with electric charges discharged from the piezoelectric elements.
In this apparatus, the potential of the electrode of each piezoelectric element is held at the bias voltage applied from the first condenser, and it is not necessary to provide, for example, a logic power source, so that the bias voltage can be obtained at a low cost by the simple configuration.
Here, it is preferable that the charger includes a second condenser charged with the electric charges.
In this apparatus, the electrode of each piezoelectric element receives the stable bias voltage from the first condenser.
Further, it is preferable that the charger includes a constant-voltage circuit which regulates a charged voltage of the second condenser, and applies the charged voltage to the first condenser.
In this apparatus, fluctuation in the charged voltage of the first condenser is suppressed. Consequently, the bias voltage applied to the common electrode of the piezoelectric element is held more constantly.
In addition, it is preferable that the second condenser is charged before a printing operation is performed.
In this apparatus, the bias voltage applied from the first condenser to the common electrode also increases so that the erroneous operation of each piezoelectric element due to the increase of the bias voltage before the printing operation is prevented.
Preferably, it is preferable that the bias power source includes: a condenser, which apply the bias voltage to the common electrode; and a charger, which charges the condenser based on a power supplied from a power source for driving the print head. The bias voltage is substantially identical with an intermediate potential of the drive signal.
In this apparatus, since the voltage difference applied between the both electrodes of the piezoelectric element comes nearly to zero, the consumed power is reduced, the voltage drop due to the natural discharge of the piezoelectric element is reduced, and the power loss is reduced.
Here, it is preferable that the charger includes a switcher, which applies the intermediate potential to the condenser when the drive signal is not used for ejecting the ink drop.
In this apparatus, the potential of the common electrode of the piezoelectric element is held at the intermediate potential by the bias voltage applied from the condenser.
Further, it is preferable that the switcher is provided as a switching element.
In this apparatus, since the switching element may be controlled by a minute signal, the switcher can be readily controlled.
In addition, it is preferable that the switcher is controlled in accordance with the drive signal.
In this apparatus, the intermediate potential of the drive signal can be readily applied to the condenser, and the condenser can be charged.
Preferably, the bias power source is provided as a reference voltage generator which applies a reference voltage having a potential which is substantially identical with an intermediate potential of the drive signal, to the common electrode.
In this apparatus, since the voltage difference applied between the both electrodes of the piezoelectric element becomes relatively low, the consumed power is reduced, the voltage drop due to the natural discharge of the piezoelectric element is reduced, and the power loss is reduced.
Further, heat generation of the piezoelectric element is reduced, so that characteristic change of the piezoelectric element due to a change in temperature decreases. Even if operation characteristic of the piezoelectric element changes due to the temperature, since the reference voltage generator holds always the potential of the piezoelectric element at the intermediate potential, temperature correction is not required.
Here, it is preferable that the head driving apparatus further comprises a charger which generates a charge signal for charging at least one of the piezoelectric elements when the drive signal is not used for ejecting the ink drop. The reference voltage generator includes: a voltage holder, which latches an arbitrary potential of the drive signal based on the charge signal; and an current amplifier, which current-amplifies a voltage output from the voltage holder.
In this apparatus, not only the desired reference voltage can be generated, but also the electrode of the piezoelectric element is charged by the relatively large current. Further, since the potential of the common electrode of the piezoelectric element can be held at the intermediate potential, it is not necessary to provide a variable power source.
Further, since it is not necessary to provide another power line, the existing circuit can be utilized as it is.
Here, it is preferable that the reference voltage is applied when the charger charges the at least one of the piezoelectric elements, based on the output voltage of the voltage holder.
In this apparatus, since the both electrodes of the piezoelectric element are respectively charged without producing the mutual voltage difference, the erroneous operation of the piezoelectric element is prevented. Consequently, charging of the piezoelectric element before the printing operation can be performed quickly.
In addition, it is preferable that the reference voltage generator discharges at least one of the piezoelectric elements when a potential of the drive signal is higher than the intermediate potential while a printing operation is performed. The reference voltage generator charges at least one of the piezoelectric elements when the potential of the drive signal is lower than the intermediate potential while the printing operation is performed.
In this apparatus, since the potential of the common electrode of the piezoelectric element is held at the intermediate potential, the bi-directional variable power source is not required.
Here, it is preferable that the reference voltage generator includes a discharger which discharges at least one of the piezoelectric elements.
In this apparatus, in a case that the potential of the piezoelectric element is higher than the intermediate potential, discharging is performed through the discharger, whereby the potential of the piezoelectric is held at the intermediate potential.
In order to obtain the above advantages, according to the present invention, there is provided a liquid jetting apparatus, comprising:
a jetting head, provided with a plurality of nozzles;
piezoelectric elements, each associated with one of the nozzles and provided with a drive electrode and a common electrode; and
the above-described head driving apparatus.
In order to obtain the above advantages, according to the present invention, there is provided a method of driving a jetting head in a liquid jetting apparatus, comprising the steps of:
providing a liquid jetting apparatus which comprises:
a jetting head, provided with a plurality of nozzles;
piezoelectric elements, each associated with one of the nozzles and provided with a drive electrode and a common electrode; and
a head driver, which generates a drive signal for driving the piezoelectric elements, and selectively supplies the drive signal to at least one of the piezoelectric elements to eject an ink droplet from at least one associated nozzle;
providing a bias power source in the liquid jetting apparatus; and
applying a bias voltage having a predetermined potential from the bias power source to the common electrode of each piezoelectric element.
Preferably, the head driving method further comprises the step of charging at least one of piezoelectric elements when the drive signal is not used for ejecting the ink drop.
Preferably, the head driving method further comprises the steps of:
determining a reference potential in the drive signal;
discharging at least one of the piezoelectric elements when a potential of the drive signal is higher than the reference potential while a printing operation is performed; and
charging at least one of the piezoelectric elements when the potential of the drive signal is lower than the reference potential while the printing operation is performed.
Preferably, the head driving method further comprises the step of varying a potential of the bias voltage so as to follow a potential of the drive signal when the drive signal is not used for ejecting the ink drops.
Preferably, the head driving method further comprises the steps of:
determining a reference potential as an intermediate potential of the drive signal; and
adjusting the bias voltage based on the reference potential.